Preserve Pose bitwise representation when accessing Pose.cache SimpleMetrics data

[klimaj]

This PR aims to make a minor change to the Pose.cache infrastructure.

Currently, accessing SimpleMetrics data in the Pose.cache dictionary via the get_sm_data method lazily initializes a SimpleMetricData entry in the Pose.data cache. If a Pose does not already contain SimpleMetrics data, then merely accessing this getter method bitwise mutates the Pose (albeit harmlessly, since it only instantiates an empty container). However, in principle, a getter ought not to modify the underlying state or binary representation of the Pose.

Herein, we instead conditionally check for Pose.data().has(CacheableDataType.SIMPLE_METRIC_DATA), performing the same test at the PyRosetta layer that the get_sm_data method performs at the C++ interface. If False, then we return an empty dictionary without lazily initializing a SimpleMetricData entry in the Pose.data cache. This minor update enables deterministic behavior when instantiating PackedPose objects, during which the Pose.cache dictionary is accessed to define the PackedPose.scores attribute. By preventing lazy initialization of the SimpleMetrics data cache, the new behavior is consistent regardless of whether input Pose objects contain SimpleMetrics data. Unit tests are also added herein to ensure consistency of the underlying bitwise representations of Pose objects with and without SimpleMetrics data.

[ajasja]

Looks good to me, love the test sequence:) (pyrosetta.io.pose_from_sequence("TEST/SIMPLE/METRIC/DATA"))

[lyskov]

LGTM!

[klimaj]

Thanks @ajasja and @lyskov !